Shown to the right is an animation illustrating one way in which cells can use the energy in ATP to drive the uptake of ions or molecules. At the beginning, the membrane transport protein is in its closed configuration which does not allow substrates or other molecules to enter or leave the cell. Next, the substrate (small red spots) binds to the membrane transport protein at the active site which is exposed to suspending medium. On the inside of the cell, ATP binds to another site and phosphorylates the "R-group" on an amino acid (usually a serine or a threonine). This covalent attachment of a phosphate group to the protein induces a conformational change in the protein such that a channel opens between the internal and the external aqueous media. Now the bound substrate enters the cell. As one molecule of substrate enters, the covalently bound phosphate comes off and the membrane transport protein returns to its closed configuration which does not allow anything to pass through the channel. Now the transport protein is ready to start the cycle again. Note that as materials are transported into the cell, ATP is used up and ADP and -PO4 accumulate. The ATP must be regenerated by glycolysis or by aerobic respiration.